1. Field of the Invention
The present invention relates to a cartridge piston comprising a surrounding side wall and being closed at its front end by a piston end face, said side wall being provided with at least one flexible sealing lip. Further, the invention relates to a combination of a cartridge piston and a cartridge.
2. Description of the Prior Art
Cartridges for compounds to be dispensed by being pressed out, such as compounds for sealing and filling purposes, consist of a tubular cartridge body into which the compound will be filled. Said cartridge body is closed on its front end, and the process of filling the compound into the cartridge body will be performed via the open rear end. Then, likewise from the rear end, the piston will be introduced into the cartridge body for sealing closure of the latter.
A cartridge piston of the type forming the basis of claim 1 is described in EP 1 845 033 A1 (Fischbach). The piston is provided with two flexible sealing lips adapted to sweep along the wall of the cartridge body so that the space containing the dispensable compound is sealed against the environment. A front sealing lip is formed with recesses allowing for release of air while the piston is being introduced, i.e. during insertion of the piston into the cartridge. On the rear sealing lip, discontinuities are formed, thus preventing a full-faced abutment of the rear lip on the inner surface of the cartridge. Said discontinuities allow for the escape of air via the rear sealing lip.
The process of applying the piston is performed in a machine at a relatively high speed. As a result, a temporarily increased pressure will be generated in the space in front of the piston end face, and this pressure has to be discharged by allowing it to move past the sealing lip, along the side wall of the piston and finally into the ambient air. A problem herein resides in that this airway offers a high flow resistance because said discontinuities leave open only a small cross-sectional area of the opening. This has the adverse effect that, in the further course of the airway on the side wall of the piston, there will not be available sufficient pressure for discharging the air. On the side wall of the piston, raised surrounding rings are formed which are in abutment with the inner wall of the cartridge and which cannot be overcome by the weak airflow. As a result, a part of the air in the region of the piston will remain entrapped within the cartridge on the outer side of the piston or also in front of the piston end face. In case that the above discontinuities are less prominent, it is also possible to insert the piston obliquely into the cartridge and to allow the air to flow past the sealing lip. Also in this case, however, the abutment of the side wall of the piston on the inner wall of the cartridge will cause a reduction of the air discharge flow. Under the influence of the resultant residual air in front of the piston, the dispensable compound will tend to harden in the cartridge and thus to become spoiled. Further, during the process of pressing the compound out of the cartridge, air bubbles in front of the piston end face will cause undesired interruptions of the pressed-out strand of filling material.
It is an object of the invention to provide a cartridge piston of the initially mentioned type which allows for an improved release of air when the piston is being introduced.